Currently, 3rd generation cellular communication systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD). In CDMA systems, user separation is obtained by allocating different spreading and/or scrambling codes to different users on the same carrier frequency and in the same time intervals. In TDD user separation is achieved by assigning different time slots to different users in a similar way to TDMA. However, in contrast to TDMA, TDD provides for the same carrier frequency to be used for both uplink and downlink transmissions. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In order to provide enhanced communication services, the 3rd generation cellular communication systems are designed for a variety of different services including packet based data communication. Likewise, existing 2nd generation cellular communication systems, such as the Global System for Mobile communications (GSM) have been enhanced to support an increasing number of different services. One such enhancement is the General Packet Radio System (GPRS), which is a system developed for enabling packet data based communication in a GSM communication system. Packet data communication is particularly suited for data services which have a dynamically varying communication requirement such as for example Internet access services.
For cellular mobile communication systems in which the traffic and services have a non-constant data rate, it is efficient to dynamically share radio resources amongst users in accordance with their needs at a particular instant. This is in contrast to services with constant data rates, where radio resources appropriate for the service data rate can be assigned on a long-term basis such as for the duration of the call.
In the current UMTS TDD standards, uplink shared radio resources may be dynamically assigned (scheduled) by a scheduler in a Radio Network Controller (RNC). However, in order to operate efficiently, the scheduler needs to have knowledge of the volume of uplink data which is waiting for uplink transmission at the individual mobile users. This allows the scheduler to assign resources to users who need them most and in particular prevents that resource is wasted by being assigned to mobile stations that do not have any data to send.
A further aspect of efficient scheduling is the consideration of user radio channel conditions. A user for whom the radio path gain to another cell is similar to the radio path gain to the current serving cell may cause significant interference in the other cell. It can be shown that system efficiency may be significantly improved if the scheduler takes into account the relative path gains from the user to each cell in the particular locale of the network. In such schemes, the power of transmissions by users for whom the path gain to one or more non-serving cells is of similar magnitude to the path gain to the current serving cell is restricted such that the inter-cell interference caused is controlled and managed. Conversely, the transmit power of transmissions by users for whom the path gain to the serving cell is far greater than that to other cells is relatively less restricted since the inter-cell interference caused by such users per unit of transmission power is less.
In practical systems, both the radio conditions and the pending data volume status may change very rapidly. In order to optimize system efficiency as these changes occur, it is important that the scheduler in the network is informed of the very latest conditions such that timely adjustment of the scheduler operation may be effected.
For example, during a typical active session, there will be periodic spurts of uplink data to send (for example when sending an email, sending completed Internet forms, or when sending TCP acknowledgements for a corresponding downlink transfer, such as a web page). These short data spurts are known as packet calls, and their duration may span from typically a few milliseconds to a few seconds. During a packet call, uplink resources are being frequently allocated and it is efficient for the buffer volume and radio channel information to be piggybacked on these uplink transmissions to continually update the scheduler as to the data sending needs of the user. However, once the packet call has completed (all data to send has been sent and the transmission buffer is temporarily empty), allocation of uplink resources is suspended. In this situation, means for informing the scheduler of the arrival of new data (at the start of a new packet call) must be found. It is important to minimize any delay in this signaling since this contributes directly to the user-perceived transmission speed.
Release 99 of the Technical Specifications for 3GPP UMTS TDD, define a layer 3 message termed the PUSCH (Physical Uplink Shared Channel) Capacity Request (PCR) message. The logical channel carrying PCR (termed the Shared Channel Control Channel—SHCCH) may be routed to different transport channels depending on the presence of available resources. For example, the PCR message may be sent on the Random Access CHannel (RACH) which is terminated within the RNC. As another example, if the resources are available, the PCR may also in some cases be sent on the Uplink Shared CHannel (USCH).
However, although this approach is suitable for many applications it is not optimal for many other applications. For example, the defined signaling is aimed at providing scheduling information to RNC based schedulers and is designed for this application, and is in particular designed with a dynamic performance and delay suited for this purpose. Specifically, the signaling is relatively slow and the allocation response by the RNC scheduler is not particularly fast due to the delays associated with communication between the base station and the RNC (over the Iub interface) and the protocol stack delay in receiving the PCR and transmitting the allocation grant message via peer-to-peer layer 3 signaling.
Recently, significant effort has been invested in improving specifically uplink performance for 3GPP systems. One way to do this is to move the scheduling entity out of the RNC and into the base stations such that transmission and retransmission latencies may be reduced. As a result, a much faster and more efficient scheduling can be achieved. This in turn increases perceived end-user throughput. In such an implementation, a scheduler located in the base station (rather than in the RNC) assumes control over the granting of uplink resources. Fast scheduling response to user's traffic needs and channel conditions is desirable in improving the efficiency of the scheduling and the transmission delays for the individual mobile stations.
However, as the efficiency of the scheduling activity relies on sufficient information being available, the requirements for the signaling functionality become increasingly severe. Specifically, the existing approach wherein signaling is transmitted to the RNC by layer 3 signaling, is inefficient and introduces delays which limit the scheduling performance of a base station based scheduler. In particular, using techniques identical to the prior art (such as the use of PCR messages) are not attractive due to the fact that the transport channels used are terminated in the RNC—the signaling information thus ends up in a different network entity than that in which the scheduler resides and an additional delay is introduced in communicating this to the base station scheduler.
For example, in a 3GPP TDD system, timely updates on radio channel conditions are especially important due to the fact that the uplink and downlink radio channels are reciprocal. As such, if the user is able to inform the network scheduler of the very latest channel conditions (as e.g. measured on the downlink), and the scheduler is able to respond with minimal delay, then the scheduler may exploit the reciprocity and assume that the radio channel conditions will be relatively unchanged by the time an uplink transmission is scheduled and transmitted. The channel conditions that may be reported by a mobile station may include the channel conditions for the cell of the scheduler but may also include channel conditions relating to other cells thereby allowing a fast and efficient scheduling taking into account the instantaneous conditions for other cells and the resulting intercell interference caused.
As another example, in 3GPP FDD systems, mobile station buffer volume status is signalled within the uplink transmissions themselves. The data is contained within the same Protocol Data Unit (PDU) as the other uplink payload data—specifically in the MAC-e PDU header. However, this means that the signaling information is dependent on the performance and characteristics of the uplink data transmissions themselves.
In particular, the uplink data transmissions are designed for efficient throughput and characteristics suited for the data being transmitted.
The delay experienced by packet data transmissions may comprise a component due to the queuing of users and a component due to the transmission itself. In a loaded system, it is common that the queuing delay is larger than the transmission delay. The capacity of the system may be improved by using the radio resources more efficiently. Higher capacity can mean that users are served more quickly, and so the queuing delay may be reduced. However, some techniques, involving one or more air interface retransmissions, afford an increase in efficiency (and hence capacity) but at the expense of transmission delay. Thus a balance must be struck between queuing delay and transmission delay in order to find a system operating point which optimizes the system performance as perceived by the end-user. Typically many packet data services are relatively delay tolerant and the communication characteristics are therefore frequently aimed at an efficient transmission of data using a minimum of the air interface resource. Consequently, link efficiency is prioritized higher than transmission delay in order to reduce queuing delay for the data traffic. However, the consequential increased transmission delay may render the approach unsuitable for carrying control signaling information to a base station scheduler and may result in an inefficient scheduling by this scheduler.
Specifically, in order to achieve an efficient communication of data bits across the air interface, retransmission of data packets which are not correctly received has been specified for most 3GPP packet data services. In such systems, data retransmissions are commonplace and hybrid and fast retransmission schemes are typically used because the optimum link efficiency (in terms of the energy required per error-free transmitted bit following retransmissions) is achieved when the probability of error for first-time transmissions is relatively high (e.g. 10% to 50%). However, the air interface transmission delay associated with a retransmission is very high as it includes the delay of the acknowledgement feedback process (e.g. the delay of waiting for a possible acknowledgement before deciding to retransmit) and of the scheduling of a retransmission data packet.
Thus, current signaling techniques are suboptimal for many base station based schedulers. For example, uplink signaling techniques adopted for 3GPP FDD uplink suffer from latencies which, if applied to a TDD uplink system, may significantly degrade the performance of that TDD system with respect to the level of performance that is achievable.
Hence, an improved signaling in a cellular communication system would be advantageous and in particular a system allowing increased flexibility, reduced signaling delay, improved scheduling, suitability for base station based scheduling and/or improved performance would be advantageous.